1. Field of Invention
This invention relates to plasma generation.
2. Description of Related Art
Generally, it is known to generate low pressure, plasma glow discharges. These low pressure, plasma glow discharges are created in a vacuum and are difficult to sustain for any length of time. Typically, the low pressure, plasma glow discharges can be maintained in a stable, uniform state.
Plasma is a term used to describe an electrically neutral, partially ionized gas composed of ions, electrons, and neutral particles. Furthermore, plasma is a phase of matter that is distinct from solids, liquids, and normal gases because plasma is produced by either high temperatures or strong constant or time varying electric fields.
Generally, discharge plasmas are produced when free electrons are energized by electric fields in a background of neutral particles, such as atoms and/or molecules. When these free electrons are energized, the free electrons collide with the neutral particles. These collisions transfer energy from the free electrons to the neutral particles and, as a result, the originally neutral background gas becomes at least partially, and in some cases fully ionized. When this occurs, the ionized gas is able to conduct currents.
A specific class of plasmas, known as current-maintained plasmas, is produced by a glow plasma discharge or an arc plasma discharge. These current-maintained plasmas are only maintained, and are only conductive, while a current is passed through the generated plasma. Thus, if the current is removed, the plasma quickly becomes nonconductive. This is because the relaxation time for most plasmas is between xcex7s and xcexcs.
The characteristics that distinguish an arc plasma discharge from a glow plasma discharge are a high gas temperature and a low cathode fan potential. However, it is possible to have a high gas temperature associated with a high cathode fall, and vice versa. Furthermore, as would be expected, there are transition points at which the characteristics of the discharge plasma change from glow plasma discharge characteristics to arc plasma discharge characteristics.
A plasma discharge encounters several intermediate stages during the transition from a glow plasma discharge to arc plasma discharge. Some of these intermediate stages are relatively stable, while other of the stages are not. However, unless the current is limited by an external method, such as, for example, a large series resistance, the final transition from a glow plasma discharge to arc plasma discharge is usually an unstable change.
Furthermore, if the current is not limited by an external method, the final transition from a glow plasma discharge to arc plasma discharge is quite rapid, and equilibrium is typically not achieved in any of the intermediate transition stages. This final transition accelerates as the pressure of the background neutral gas increases towards atmospheric pressure.
There are two basic methods that can be used to generate discharge plasmas. The first method is through external ionization. External ionization produces plasmas by using photons or charged particles. However, this method produces a weakly ionized gas at high (atmospheric) pressure. Additionally, the efficiency of external ionization is rather low, and therefore the cost of such a method relatively high.
The second method that can be used to generate plasmas is internal ionization The method of internal ionization generates electrons and ions in a self-sustained gas discharge. If the current is strongly limited, or the applied voltage is smaller than the breakdown voltage, this method can produce large volumes of weakly ionized gas. However, if the current is not limited and the voltage is high enough, there is a transition to an arc.
For example, U.S. Pat. No. 5,939,829 to Schoebach et al. merely discloses a discharge device for operation in gas at a prescribed pressure including a single cathode having a plurality of microhollows, and a single anode spaced from the cathode.
Additionally, U.S. Pat. No. 6,005,349 to Kunhardt et al. merely discloses a method and apparatus for stabilizing glow plasma discharges by suppressing the transition from glow-to-arc. The Kunhardt et al. apparatus includes a single upper electrode, a single cathode, a collar, and a perforated dielectric plate positioned over the cathode. The holes in the perforated dielectric plate provide a narrow channel to limit the overall current density. Additionally, the collar is positioned between the upper electrode and the cathode to contain the glow discharge.
Thus, methods for generating large volumes of ionized gas at high pressure typically involve ionization mechanisms wherein the energy required for the ionization is drawn from electrical energy. Examples of this kind of ionization mechanism are radio frequency (RF) and microwave discharges, barrier discharges, and pulsed corona discharges where the discharge is sustained by either alternating or pulsed fields, and steady state discharges wherein the discharge is driven by a direct current (dc) power source.
Much of the efforts in generating stable glow discharges at high pressure have focused on preventing the onset of instabilities in the regions near the electrodes, particularly in the cathode region. These regions near the electrodes are regions of higher electric field and consequently higher power density compared to the positive column of the discharge. These regions are, therefore, a cradle of instabilities, which lead to constrictions and arc formation in the discharge. The glow-to-arc transition (GAT), the development of a highly conductive channel, which shorts out the glow discharge, shows the first visible evidence near the cathode. Other instabilities which may develop in the positive column of discharges in electronegative gases, such as the attachment instability, are generally more benign than the GAT.
Segmentation of the cathode, and ballasting the individual discharge resistively has been used to prevent the onset of the GAT instability in atmospheric pressure glow discharges. The current density in the bulk of the glow discharge is known to increase linearly with pressure, whereas the current density in the cathode layer for normal mode operation increases quadratically with pressure.
In order to make the conditions in the bulk and at the cathode compatible, the current cross-section at the cathode must be reduced with increasing pressure. This was achieved by using ballasted pins as individual cathodes with cross-sections that are small compared to the area of the cathode segment. The onset of bulk instabilities can be prevented by flowing air with such a speed through the discharge that the plasma is replaced by cold air on a time scale that is small when compared to the inverse of the growth rate of the bulk instabilities.
None of these previous efforts disclose all of the benefits of the present invention, nor does the prior art teach or suggest all of the elements of the present invention.
None of the known methods for producing a glow discharge produce a high pressure, uniform, stable plasma glow discharge. Specifically, none of the known methods for producing a glow discharge produce a dc driven, high pressure, uniform, stable plasma glow discharge with relatively high electron densities, such as, for example, 1013 cmxe2x88x923 in air. It should be understood that the term xe2x80x9chigh pressurexe2x80x9d refers to pressures that are approximately atmospheric pressures and could range from between slightly less than one atmosphere to several atmospheres.
Research on high pressure glow discharges is motivated by applications such as instantly activated reflectors and absorbers for electromagnetic radiation, surface treatment, thin film deposition, remediation and detoxification of gaseous pollution and gas lasers.
Therefore, a direct current high pressure glow discharger, according to this invention, allows simultaneous generation of relatively high electron densities at relatively low temperaures with stable, direct current, homogenous glow discharge plasma at relatively high pressure.
The direct current high pressure glow discharger, according to this invention, includes a microhollow cathode, a microhollow anode, and an additional anode spaced apart from the microhollow anode. The microhollow cathode and the microhollow anode are separated by a dielectric and a borehole is formed through the microhollow cathode, the microhollow anode, and the dielectric.
In various exemplary embodiments, the electrode distance of the microhollow cathode discharge is approximately equal to the dimension of the borehole. The microhollow cathode of this invention produces discharges that operate at low current in a Townsend mode, where in an electrode configuration the electric field is dominantly axial.
When current is increased in the direct current high pressure glow discharger, the microhollow cathode discharges transfer into the microhollow cathode mode with high radial electric fields in the cylindrical cathode fall of the discharge. The axial field in the plasma column, which serves as a virtual anode in this case, is rather small. When the microhollow cathode discharge (MHCD) operates in this mode, external fields generated by the additional, positively biased electrode in front of the microhollow anode can penetrate into an electrode cavity and force the microhollow cathode current to flow to the additional electrode.
In this manner, the MHCD serves as an electron source for a glow discharge between a microhollow anode and the additional electrode. Thus, the microhollow cathode system, according to this invention, is considered as plasma cathode, and the additional electrode is considered an anode of a microhollow cathode sustained (MCS) glow discharge.
In the plasma cathode sustained glow discharge, of this invention, the cathode fall is eliminated. Thus, the glow discharge is stable as long as the MHCD is stable, providing that the conditions in the main discharge are such that bulk instabilities are avoided.
This invention allows the MHCD current to determine the glow discharge current at a constant voltage across a main glow discharge gap. Additionally, the MHCD current can be controlled by the MHCD voltage. For example, small variations in the MHCD voltage produce large swings in the anode current. Thus, the MCS glow discharge can be used to generate patterns by individually controlling discharges in various discharge arrays.
Accordingly, this invention separately allows the threshold current required for the onset of the MCS glow discharge to activate a large volume glow discharge with relatively small voltage swings in the MHCD voltage. The relative magnitude of the voltage and the voltage swing is determined by the particular gas that is used. Therefore, when the direct current high pressure glow discharger, of this invention, operates below the threshold for onset of the MCS glow discharge, only a relatively small voltage pulse is required to turn the main discharge on. Once in the on-state, the direct current high pressure glow discharger will stay in the on state, even when the voltage pulse is turned off. This is because of the hysteresis of the main discharge.
The concept of the MHCD sustained high-pressure glow discharges can be applied to any gas or gas mixture, such as, for example, air and/or molecular and electronegative gases.
Thus, this invention produces a direct current, stable plasma discharge in atmospheric pressure air.
Thus, this invention separately elimates the conditions for glow-to-arc transition in the cathode fall region of a glow discharge by eliminating the cathode fall.
This invention separately replaces the cathode with an externally controlled electron emitter that provides electrons through external sources rather than through ion impact at the cathode.
This invention separately allows the electron emission to be adjusted to a large volume glow discharge.
This invention separately reduces the thermal losses in the cathode and reduces the requirements for cooling.
This invention separately provides microhollow cathode discharges that operate at low current in a Townsend mode.
This invention separately improves the stability of a high-pressure glow discharge.
This invention separately provides a MHCD that serves as a current valve for the glow discharge.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of various exemplary embodiments of the systems and methods of this invention.